1. Field of the Invention
This invention relates to electrolysis of aqueous alkali metal chloride solutions.
2. Description of the Prior Art
The electrolysis of an aqueous alkali metal chloride solution has as its primary products chlorine and alkali metal hydroxide. A secondary product is alkali metal chlorate. Generally, chlorate formation is considered unfavorable except where the chlorate is desired to be recovered as a by-product of the electrolysis reaction.
In prior art electrolytic cells equipped with a selectively permeable membrane barrier between the anode and cathode compartments of said electrolytic cell, it was believed that chlorate formation was dependent upon the amount of hydroxide ion migrating from the cathode compartment to the anode compartment since chlorate formation occurs in the anolyte according to the following equation: EQU 3OH.sup.- + 3Cl.sub.2 .fwdarw. 3Cl.sup.- + 2HCl + HClO.sub.3
it was believed that since the migration of hydroxide ion from the catholyte across the membrane into the anolyte depends primarily upon the alkali metal hydroxide concentration in the catholyte that reducing the amount of hydroxide ion migrating into the anolyte by operating the electrolytic cell at a low concentration of alkali metal hydroxide in the catholyte would reduce alkali metal chlorate formation in the anolyte.
Actually the formation of chlorates proceeds in two steps. In the first step, hypochlorous acid is formed by an equilibrium reaction: EQU Cl.sub.2 + H.sub.2 O .revreaction. HCl + HClO
in the second step, the hypochlorous acid disproportionates to chlorate and chloride in accordance with the following equation: EQU 3HClO + 3NaOH .fwdarw. NaClO.sub.3 + 2NaCl + 3H.sub.2 O
the second reaction is irreversible and rate determining.
In the first reaction, as can be seen, the formation of hypochlorous acid and hence chlorates would be suppressed by the addition of HCl to the anolyte. It is known to maintain the pH of the anolyte at a pH of less than 3 by the addition of hydrochloric acid so as to suppress chlorate formation. This is taught in U.S. Pat. No. 3,948,737. In this patent there is disclosed a process for the electrolysis of brine in which the formation of sodium chlorate in the anolyte is minimized preferably by maintaining the pH of the brine solution in the anolyte within the range of about 2.5 to 4. In this patent there is also disclosed the introduction of water into the catholyte so as to maintain the sodium hydroxide concentration of the catholyte not in excess of about 33% by weight.
In the prior art electrolytic cells utilizing an asbestos diaphragm as a barrier separating the anode compartment from the cathode compartment, the migration of hydroxide ions from the cathode compartment to the anode compartment is counteracted by the steady hydraulic flow of anolyte liquid across the diaphragm so as to effect a backwashing of the hydroxide ions away from the diaphragm thus tending to keep the hydroxide ions in the cathode compartment where they are formed. In the diaphragm cells, the formation of chlorates can be kept at a minimum by properly choosing the cell operating conditions such that by maintaining the salt conversion in the anolyte at a concentration of 50% or below, adequate reduction in chlorate formation is effected. For instance, at 50% alkali metal chloride conversion in the anolyte compartment of the diaphragm cell, the formation of chlorate is 0.25 gram per liter. As the salt conversion in the anolyte is increased to 55%, the chlorate formation increases to 0.5 gram per liter and upon increasing the salt conversion beyond 55% the chlorate formation increases very rapidly.
It is known that in a cell specifically designed to produce alkali metal chlorates, the anolyte and catholyte are mixed, thus dispensing with the diaphragm or mercury cathode of prior art chlor-alkali electrolytic cells. For instance, U.S. Pat. No. 3,623,967 discloses an electrolytic apparatus for the production of alkali metal chlorate.
In the membrane-type electrolytic cells for the electrolysis of brine to produce chlorine and sodium hydroxide, a so-called "perm-selective" barrier is used consisting, for instance, of a hydrolyzed copolymer of tetrafluoroethylene and a sulfonated perfluorovinyl ether. Such polymers are disclosed in U.S. Pat. No. 3,282,875.
Other membranes have been developed, specifically the perfluorocarboxylic acid type membrane of Asahi Chemical Industry Company, Limited, and the hydrocarbon type cation exchange membrane. Modification of these and other ion exchange membranes are currently being made. The copolymers of tetrafluoroethylene and sulfonyl fluoride perfluorovinyl ether utilized as an ion exchange membrane in such electrolysis cells are sold under the trademark "Nafion."